1.0 Field of the Invention
The present invention relates generally to gas turbine engines and, more particularly, to a blade retainer sub-assembly for use in gas turbine engines.
2.0 Related Art
Conventional high bypass ratio turbofan engines, which are included in the more general category of gas turbine engines and which may be used for aircraft propulsion, typically include a fan, booster, high pressure compressor, combustor, high pressure turbine and low pressure turbine in serial axial flow relationship about a longitudinal centerline axis of the engine. The high pressure turbine is drivingly connected to the high pressure compressor via a first rotor shaft, and the low pressure turbine is drivingly connected to both the fan and booster via a second rotor shaft. The fan includes an annular disk, rotatable about the longitudinal centerline of the engine, and a plurality of radially extending blades mourned on the disk. Each blade has a relatively large radial dimension and typically includes an airfoil, a platform and a radially inner dovetail. Radial retention of the blades is commonly achieved by inserting the dovetail of each blade into a mating axially extending dovetail slot formed in the disk, wherein the dovetail slots are equally spaced about the disk rim and wherein pressure faces of the blade dovetail are forced against mating faces of the dovetail slots during engine operation. In high bypass ratio turbofan engines, having blades with a relatively large radial dimension, the blades may also include interlocking mid-span shrouds to prevent excessive blade deflection and to dampen blade vibration. With shrouded blades, it is common to utilize oversized dovetail slots to facilitate blade removal, wherein the radial height of the dovetail slot is greater than the radial height of the blade dovetail thereby allowing the blade to be moved radially inward in the slot and disengaged from the shrouds of adjacent blades. In this instance radial spacers are commonly inserted between the inner surface of the blade dovetail and the bottom of the dovetail slot to keep the dovetail pressure faces in abutting relationship with the mating faces of the dovetail slot so as to prevent undesirable blade clanking and the associated dovetail pressure face coating wear during periods of windmilling.
In addition to radial retention, axial retention of fan blades has been a continuing problem for the industry. Although axial retention is required in both directions, the following discussion focuses on axial retention in the forward direction, which is the subject of the present invention. During engine operation, a fan blade may encounter foreign objects such as birds or debris picked up along a runway, or more infrequently, a failed blade or blade fragment may be released wherein a trail blade may impact the released blade or blade fragment. In either case, when a blade encounters a foreign or domestic object, the affected blade pushes the object aft due to the blade camber. This results in a reaction force, which can be relatively high in the case where a blade impacts a released blade or blade fragment, which tends to force the affected blade out of the dovetail slot in a forward direction. Interaction between rotating and non-rotating components, which may be caused by excessive blade tip rubs, can result in an induced rotor system vibratory response which presents another scenario which can result in relatively high axial loads which tend to force one or more of the blades out of the corresponding dovetail slots in an axially forward direction.
One prior method for providing axially forward retention of fan blades in gas turbine engines may be referred to as the "double bar system." In this system a pair of bars are inserted between the inner surface of the blade dovetail and the bottom of the dovetail slot, on either side of a radial spacer. Each bar has an enlarged aft portion which hooks around the aft face of a disk post positioned adjacent to the dovetail, and an enlarged forward portion which engages the forward surfaces of both the blade dovetail and the disk. After assembly, the two axial retention bars and the intervening radial spacer are then bolted together, wherein radial retention of the bars and the spacer is provided by the positioning of each element under the blade dovetail and the conventional radial retention of the blade. While the "double bar system" has proven to be an effective method of axially retaining fan blades, in both directions, the "double bar system" is not weight efficient.
Another prior method for providing axial retention, in a forward direction, of fan blades in gas turbine engines may be referred to as the "shear plate system" which is more weight efficient than the "double bar system." With the "shear plate system," axially extending lugs protrude from the forward face of each disk post wherein each lug includes a pair of radially extending and circumferentially facing slots formed in each circumferential end of the lug. Each lug slot is open adjacent to the dovetail slot such that opposing slots in adjacent lugs may slidingly accept a shear plate. The shear plate closes off the forward end of the dovetail slot and comprises a generally rectangular cross-section. The circumferentially facing ends of the shear plate extend along generally radial lines and are generally parallel such that the cross-section of the plate is generally constant along the radial height of the plate. The shear plate further comprises a flat axially aft facing surface and a mount bracket protruding from an otherwise flat axially forward facing surface. The mount bracket includes an enclosed interior hole which accepts conventional fastening means for fixedly attaching the bracket to a radial spacer positioned under the blade dovetail, thereby providing radial retention of the shear plate. The flat axially aft facing surface spans the full circumferential width and radial height of the blade dovetail such that when large reaction forces tend to drive the blade forwardly out of the dovetail slot the entire forward face of the blade dovetail contacts the flat axially aft facing surface of the shear plate. The axial force from the blades is reacted through the plate into the disk lugs at the areas of contact between the shear plate and the slots formed in the disk lugs.
Although the "shear plate system" has proven to be an effective retention system for certain applications, an analysis and component testing conducted by the present inventors associated with a potential application for larger turbofan engines, having significantly higher axial load requirements, revealed the following problems associated with the prior "shear plate system" which remained unsolved prior to the present invention. One problem with the prior "shear plate system" is that both the shear plate and the two adjacent disk lugs which contact the shear plates are loaded unevenly due to the application of axial forces from a fan blade for the following reasons. With the exception of the axial location corresponding to the circumferentially facing slots formed in the disk lugs, the contour of the disk lugs is determined by the shape of the dovetail slots formed in the disk. Since the shear plate spans the circumferential width and radial height of the dovetail and since the aft surface of the plate is flat, the full forward face of the blade dovetail contacts the flat aft surface of the plate when the axial force is applied to the blade. As the axial force is reacted through the plate, the center of the plate deflects which causes the load to be transferred to the portion of the plate corresponding to the edges of the dovetail around the dovetail periphery, wherein the loads are then reacted through circumferentially adjacent portions of the disk lugs. Since the outer contour of the disk lugs is determined by the dovetail slots, the contour of the lugs is nearly circumferentially aligned with the blade dovetail in the area corresponding to the dovetail pressure faces (one on each circumferential side of the dovetail) and is circumferentially offset from the blade dovetail contour at locations radially outward and radially inward from the dovetail pressure faces. This allows the shear plate edges to deflect in areas above and below the dovetail pressure faces which in turn causes a disproportionate amount of the axial force to be reacted in the portion of the plate adjacent the dovetail pressure faces where the local deflection in the plate is relatively limited. The aforementioned near circumferential alignment between the dovetail pressure faces and the adjacent areas of each of the two corresponding disk lugs creates a "scissoring" action which results in a primarily shear load being applied to the corresponding areas of the plate. The applied shear load can be relatively high due to the aforementioned uneven load distribution resulting in a premature shear failure of the plate, i.e., a failure corresponding to a significantly lower axial force than would be predicted based upon an even load distribution in the plate. The aforementioned "scissoring" action also causes the disk lugs to be unevenly loaded which can result in a premature failure of the lugs.
Another problem with the prior "shear plate system" is that the mount bracket, which protrudes from the forward surface of the shear plate and which includes an enclosed interior hole, locally increases the bending stiffness of the shear plate which further compromises the ability of the shear plate to evenly distribute the reaction forces over the contact area with the disk lugs.
The lug slots which accept the shear plate define a minimum circumferential width of the lug cross-section throughout the radial height of the disk lugs. The fact that the circumferential ends of the shear plate are generally parallel and extend along generally radial lines creates yet another problem with the prior "shear plate system" since the configuration of the circumferential ends of the shear plates results in an inadequate circumferential width of the disk lugs in the area of highest loading.
In view of the foregoing, prior to this invention a need existed for an improved blade retaining sub-assembly to resolve the problems associated with prior retention systems and thereby provide increased capability for axial retention of blades in a cost and weight efficient manner.